The treaty of Saint-Clair-sur-Epte (911) is the foundational document of the Duchy of Normandy, establishing Rollo, a Norse warlord and Viking leader, as the first Duke of Normandy in exchange for his loyalty to the king of West Francia, following the Siege of Chartres. The territory of Normandy centered on Rouen, a city in the Marches of Neustria which had been repeatedly raided by Vikings since the 840s, and which had finally been taken by Rollo in 876.